A user having a network access device (mobile host), who moves from a location covered by the user's home network to another location covered by a foreign access network, can be characterized as “roaming” between networks. Typically, the complicated handoff procedure that enables the mobile host to gain access to the new network can cause QoS problems. When the mobile host moves from the coverage of one network and roams into the coverage of the new or “arriving” network, three basic actions occur to effectuate a handoff: (1) the mobile host authenticates to the arriving network; (2) the arriving network allocates resources for the mobile host; and (3) the departing network releases any resources allocated to the mobile host. During the handoff process, some data packets that were forwarded to the mobile host from the mobile host's home agent can be dropped. Typically, this occurs when the home agent has not yet been updated with a new care-of IP address for the mobile host corresponding to the arriving network, and packets are still being routed to the departing network at the old care-of IP address. The home agent will route packets to the old care-of IP address until it is updated with a new care-of IP address. Since the mobile host or a foreign agent on the network provides the home agent with the new care-of IP address after the hand-off process is complete, packet loss is likely. This can occur during handoffs between different types of access networks or between access networks under different administrations.
One proposed solution to this problem is referred to as “bicasting.” See e.g., Koichi Ishibashi, Keiichi Shimizu, and Shoichiro Seno, “Behavior of A Mobility Agent in Mobile IP in order to manage the flow,” Internet Draft, October 2001, see the Internet Engineering Task Force web site, for example at www.ietf.org/internet-drafts/draft-ishi-mobileip-behavior-ma-00.txt. In bicasting, a mobile host or a foreign agent on the departing access network updates the home agent with a new care-of IP address before the handoff procedure is initiated. This action triggers the home agent to forward incoming packets to each of the arriving and departing access networks. The mobile host (or foreign agent on the arriving access network) subsequently notifies the home agent after the handoff process is complete. In response, the home agent stops bicasting incoming packets. If the mobile host is capable of receiving only one wireless signal at a time, bicasting requires foreign agents being deployed on the access networks. If the mobile host is capable of simultaneously receiving multiple wireless signals, the deployment of a foreign agent is not required. The bicasting technique was designed for low-latency applications. Two round trips of signaling messages are necessary to effectuate a handoff: one to indicate when the handoff begins, and the other to indicate when the handoff ends. Consequently, if a blackout period occurs during the handoff procedure, packet loss will be inevitable if the foreign agent cannot buffer the packets. Moreover, packet loss may occur if the departing access network unexpectedly fails, thereby depriving the mobile host of any opportunity to notify the home agent that handoff to the arriving network has started. Although lost packets can be recovered by a transmission control protocol (TCP) if they belong to a TCP connection, such loss negatively impacts the throughput of the TCP connection. The TCP will interpret the packet loss due to network congestion, and will thus slow the packet transmission.
Another technique is known as “re-tunneling.” See, Govind Krishnamurthi, Robert C. Chalmers, and Charles E. Perkins, “Buffer Management for Smooth Handovers in IPv6”, Internet Draft, July 2000, http://www.iprg.nokia.com/˜charliep/txt/mobilebuf/buffer6.txt. In re-tunneling, the home agent, mobile host or a foreign agent (with a suitable buffer) on the arriving access network, provides the new care-of IP address to the foreign agent on the departing network, such that it can re-tunnel incoming packets to the new care-of IP address. The re-tunnel procedure has been proposed for low-loss applications under the assumption that the transmission delay between two foreign agents is significantly smaller than the delay between a foreign agent and a home agent. Using this methodology, two round trips of signaling messages are required for handoff: the first to send the new care-of IP address to the home agent, and the second to send the new care-of IP address to the foreign agent on the previous access network. As with bicasting, packet loss can occur if the departing access network unexpectedly fails or corrupts, thereby depriving the mobile host of any opportunity to notify the foreign agent that handoff to the arriving network has started.
Another method referred to as “retrieve-and-resend,” is similar to retunneling discussed above. However, here the re-tunnel procedure is performed by the home agent in lieu of the foreign agent.
In typical IP mobility configurations where the mobile host is capable of receiving only one wireless signal, the foregoing methods are unsuitable without support from foreign agents.